Electron image device having target comprising porous region adjacent conductive layer and outer, denser region

ABSTRACT

This invention relates to such electron image devices as television camera tubes and image intensifier tubes and includes in one illustrative embodiment an electrically conductive member upon which there is disposed a first layer or region of a secondary emissive material deposited in a porous form to allow conduction of the secondary electrons through the voids of the porous material, and a second layer or region of greater density than the first layer disposed upon the first region to inhibit the escape of the secondary electrons emitted within the volume of the first layer.

United States Patent Goetze et al.

11 1 3,657,596 1 Apr. 18, 1972 ELECTRON IMAGE DEVICE HAVING TARGETCOMPRISING POROUS REGION ADJACENT CONDUCTIVE LAYER AND OUTER, DENSERREGION Gerhard W. Goetze, Elmira; Alvin H.

[72] inventors:

Boerio, Horseheads Township, Elmira,

both of NY.

[73] Assignee:

sburgh, Pa.

May 20, 1965 Filed:

Appl. No.:

References Cited UNITED STATES PATENTS Westinghouse ElectricCorporation, Pitt- I 2,927,254 3/1960 Kazan ..3 13/65 A 3,001,098 9/1961Sc'hneeberger ....313/l03 X 3,002,124 9/1961 Schneeberger ....313/103 X3,128,406 4/1964 Goetze et al....,... ..3l3/65 3,197,662 7/1965Schneeberger ..313/104 3,213,315 10/1965 Lempert ..313/65 A X FOREIGNPATENTS OR APPLICATIONS 715,447 9/ i 954 Great Britain ..3 13/66 879,56911/1961 Great Britain .;...3l3/65 Primary Examiner-Robert Sega]Attorney-F.1d. Henson and C. F. Renz ABSTRACT This invention relates tosuch electron image devices as television camera tubes and imageintensifier tubes and includes in one illustrative embodiment anelectrically conductive member upon which there is disposed a firstlayer or region of a secondary emissive material deposited in a porousform to allow conduction of the secondary electrons through the voids ofthe porous material, and a second layer or region of greater densitythan the first layer disposed upon the first region to inhibit theescape of the secondary electrons emitted within the 2,678,400 5/ 1 954McKay ..3 13/89 X vomme f the fi t layer 2,757,233 7/1956 Webley....3l3/65 A 2,905,843 9/1959 Lubszynski ..3 13/65 A 12 Claims, 5Drawing Figures ELECTRON IMAGE DEVICE HAVING TARGET COMPRISING POROUSREGION ADJACENT CONDUCTIVE LAYER AND OUTER, DENSER REGION Goetze andBoerio, titled Image Storage System, and assigned to the assignee ofthis invention. The target member therein described comprises aconductive plate made of a suitable material such as aluminum onto whichthere is deposited a layer of porous material capable of generatingsecondary electrons in response to a bombardment of high energy primaryelectrons. The above-mentioned copending application further describesan application of this target member as a storage element in atelevision camera tube, in which an input radiation image is convertedby a 'photocathode element into an electron image and directed onto thetarget member. The resulting stored image on the target member is readout in the form of a video signal representative of the radiation imagedirected onto the photocathode element. The video signal may then bedisplayed on a conventional display device. As described in theabove-mentioned copending application, the readout of the informationrecorded upon the target member is achieved by directing a low energyelectron beam upon the target member. Due to the erratic motion of thelow energy reading beam, it has been often found necessary to dispose agrid electrode adjacent to the surface of the target member forcollimating the electrons into a path substantially normal to thesurface of the target member. Typically, a voltage in the range of 200to 400 volts may be applied to this grid electrode.

in operation, the target member is polarized by applying a potential tothe conductive plate and scanning the porous layer with an electron beamemitted by a suitable electron gun. The potential applied to theconductive plate is set at a suitable value of approximately voltspositive with respect to the cathode element of the electron gun whichis normally placed at ground potential. The electron image (of primaryelectrons) emitted by the photocathode element is accelerated to anenergy (about 10 keV) sufficient to penetrate the conductive plate andto be directed within the porous layer, thereby, dissipating the energyof the primary electrons and creating many low energy secondaryelectrons. Most of the low energy secondary electrons are directed underthe influence of the electric field established by the polarization ofthe porous layer through the voids of the porous layer and are collectedby the conductive plate. The flow of secondary electrons establishes asecondary electron conduction current (i.e., SEC current), which drivesthe exposed surface of the porous layer progressively positive until thepotential of this surface is essentially equal to that of the conductiveplate. A pattern of charges may thus be established on the exit surfacedue to the secondary electron conduction. The intensity of the chargesis a function of the number and energy of the primary electrons, thestrength of the electric field established by polarization, and thecapacity of the storage layer. Further, the resultant pattern of chargescorresponds to the image of the primary electrons.

The target member has the further property of emitting from its exitsurface secondary electrons which are accelerated by the externalelectric field established by the collimating grid electrode of theelectron gun. The secondary elec trons which are emitted from the targetmember are designated as transmitted secondary electrons (i.e., TSE).The transmitted secondary electrons serve to increase the potential ofthe exit surface beyond that potential established upon the conductiveplate to an equilibrium value between the potential of the gridelectrode and the conductive plate. This equilibrium potentialestablishes an electric field acrossthe porous layer, opposite inpolarity to that of the initial polarization, which is normally greatenough to cause breakdown of the porous layer of the target member. Evenif the intensity or duration of the bombarding primary electrons islimited to halt this process before breakdown potential of the porouslayer is exceeded, the target is still not safe from destruction if theexit surface of the target member is permitted to charge to a valuegreater than the reflective first crossover of the storage material ofthe porous layer. In the process of reading out the pattern of chargesestablished on the target member, a low energy electron beam emitted bythe electron gun will cause reflective secondary emission from thetarget member. When this phenomena occurs, the emitted secondaryelectrons generated by the low energy beam of electrons will beattracted by the high potential placed upon the grid electrode, and thesurface of the target member will become increasingly positive due tothe loss or subtraction of electrons. The potential of the exit surfaceof the target member will continue to be driven positive until it isessentially at the potential of the grid electrode disposed adjacent tothe target member. However, the high potential applied to the gridelectrode and established upon the surface of the target member isnormally sufficient to cause breakdown of the storage material of thetarget member. As disclosed in the above-mentioned copendingapplication, the destruction of the target member may be prevented bythe insertion of a second or auxiliary grid between the first gridelectrode for collimating the reading electron beam and the targetmember. A positive potential below the first crossover of the storagematerial of the target member is applied to the auxiliary grid electrodeto thereby prevent the surface of the target member from assuming avoltage which would cause the breakdown of this member.

Because of the relatively low first crossover potential for mostpractical materials, it is desirable to maintain the auxiliary gridpotential at a low potential with respect to the conductive plate of thetarget member. Under these circumstances, the electric field establishedbetween the conductive plate and the auxiliary grid is extremely low sothat substantially no transmission secondary electrons (TSE) are emittedfrom the exit surface of the porous layer, and that the target member ischarged essentially due to the operation of secondary electronconduction current (SEC).

Another application of this target member is disclosed in U.S. Pat. No.3,128,406 by Goetze and Kanter. in this application, the target memberis employed in a direct view imaging tube in which an input radiationimage such as light is directed onto an input screen and an output imageis displayed on an output screen with increased brightness and/orcontrast. In the above-mentioned US. patent, the target members (ordynodes) are inserted between the input and output screens to therebymultiply'the electrons emitted by the input screen in response to theinput radiation. An auxiliary grid may be inserted in such an image tubeto accelerate the emitted secon dary electrons and also to limit thepotential to which the surface of the porous layer of the target membermay rise. As disclosed in the above-mentioned U.S. patent, a pluralityof the target members may be inserted between the input and outputscreens to thereby repeatedly multiply the electrons emitted by theinput screen. In order to direct the electrons from one target member tothe next, an increasingly higher or more positive potential is appliedto successive target members. It may be understood that unless anauxiliary grid is employed, the target member may be destroyed due tothe increasing emission of secondary electrons and the resultant rise ofthe exit surface of the porous layer toward the potential of theadjacent electrode. It is noted that the establishment of the potentialof the exit surface of the porous layer is due primarily to the emissionof transmission secondary electrons (TSE). An auxiliary grid is requiredto limit the potential to which the exit surface of the porous layer mayrise. in order to prevent the destruction of the porous layer of thetarget member, the

auxiliary grid should be held at a voltage such that the electric fieldthus established between the target member and the auxiliary gridelectrode does not exceed about 150 volts per cm.

However, the insertion of an auxiliary grid electrode in either a directview imaging tube or in a television camera tube has been foundobjectionable for the following reasons. First, the insertion of such agrid electrode requires not only the incorporation of additionalelements and their mounting within the tube, but also the provision ofthe associated equipment for establishing the grid electrode at thecorrect potential. Second, the pattern of information to be derived froma target member of such a device will be degraded by the incorporationof such a grid electrode.

Accordingly, it is an object of the present invention to provide animproved image device and target member therefor.

A further object of this invention is to provide an improved electronimage device wherein the requirement of an auxiliary grid electrode tomaintain the surface potential of the target member of this device belowthe breakdown and/or first crossover level is eliminated.

A still further object of this invention is to provide an improvedtarget member for an electron image device wherein a surface potentialestablished upon the target member may be controlled by the nature ofthe structure of the target member itself.

A further object of this invention is to provide a target member for anelectron image device which may be produced by a simple manufacturingprocedure at a low cost.

Briefly, the objects of this invention are accomplished by providing anelectron image device in which an improved target member may beincorporated having the characteristic that the surface potentialthereof may be controlled by the structure of the target member itself.More specifically, the target member is constructed having a firstporous layer or region of a material having the property of emittingcopious secondary electrons and a second region thereon of a denserconsistency for reducing the escape probability of transmissionsecondary electrons from the surface of the target member. In oneembodiment of this invention, the regions or layers of the target membermay be established as discrete layers; whereas in a second embodiment ofthis invention, the target member may be constructed as a unitary layerwherein the density of the layer varies from a porous region upon onesurface to a denser region upon the other surface.

Further objects and advantages of the invention will become apparent asthe following description proceeds and features of novelty whichcharacterize the invention will be pointed out in particularity in theclaims annexed to and forming a part of this description.

For a better understanding of the invention, reference may be had to theaccompanying drawings, in which:

FIG. 1 is a diagrammatic view of a camera device embodying thisinvention;

FIG. 2 is a diagrammatic view of a direct view imaging device embodyingthis invention; and

FIGS. 3, 4, and 5 are enlarged sectional views of the target memberswhich may be utilized in either of the devices shown in FIGS. land 2.

Referring in detail to the drawings and in particular to FIG. 1, anillustrative embodiment is shown wherein the teachings of this inventionmay be incorporated within a television camera tube 40. The camera tube40 comprises an envelope 42 made of a suitable insulating materialhaving one end thereof enclosed by a faceplate 44. The faceplate 44 isdesigned to be transmissive to the desired radiation from a scene 62 andis made of a suitable material such as glass in the case of a visiblelight input. A photocathode 45 is provided on the interior of thefaceplate 44 and is made of a photoemissive material sensitive to theinput radiation such as cesium antimony for a visible light input. Anelectron gun 50 is provided at the opposite end of the envelope 42 forgenerating and forming a pencil type electron beam which is directedupon a target member 24. The electron gun 50 is of any suitable type forproducing a low velocity pencil-like electron beam and may consist of acathode element 52, a control electrode 54, and an acceleratingelectrode 55. The electrodes 52, 54, and 55 of the electron gun 50 alongwith a field electrode 56 formed as a coating upon the interior of theenvelope 42 provide a focused electron beam which is directed upon thetarget member 24. Deflection means 58 illustrated as a coil is providedaround the envelope 42 for deflecting the electron beam. By applicationof a suitable potential, the low energy electron beam emitted by theelectron gun 50 is scanned over the surface of the target member 24 in aconventional manner. A focusing means 60 illustrated as a coil is alsoprovided about the envelope 42 to provide focusing of the electron beamemitted from the electron gun 50 onto the target member 24. In addition,the focusing means 60 also focuses the photoelectrons emitted from thephotocathode 45 onto the target member 24.

The target member 24 is disposed within the envelope 42 between theelectron gun 50 and the photocathode element 45. Between the targetmember 24 and the photocathode element 45, there are provided aplurality of electrodes illustrated as 46 and 48 with suitablepotentials provided thereon for accelerating and focusing the electronsemitted by the photocathode element 45 onto the target member 24.Positioned between the target member 24 and the electron gun 50, thereis provided a mesh or grid 64 adjacent to and parallel to the targetmember 24. In one illustrative embodiment of the invention, the grid 64is made of a fine wire mesh of an electrically conductive material suchas nickel and is placed approximately 0.050 inches from the surface ofthe target member 24. A potential of between 200 to 400 volts is appliedto the grid 64 for collimating the electrons emitted by the electron gun50 into a path substantially normal to the surface of the target member24.

Further, the target member 24 is mounted within the envelope 42 by asupport ring 66 made of a suitable material such as Kovar (a trademarkof the Westinghouse Electric Corporation for an alloy of nickel, ironand cobalt). The structure of one suitable target member 24 is furtherillustrated in FIG. 3. The target member 24 includes a support layer 27of a suitable insulating material such as aluminum oxide with anelectrically conductive member or layer 26 deposited thereon of asuitable material such as aluminum. A continuous, porous layer or region28 of a suitable insulating, secondary emissive material such aspotassium chloride is provided on the conductive layer 26. Othersuitable materials for use in accordance with the present invention arebarium fluoride, lithium fluoride, magnesium fluoride, magnesium oxide,cesium iodide, and sodium chloride. The layer 28 as will be described indetail later is deposited as a smoke or porous type layer. A secondlayer or region 30 is disposed on layer 28 and is characterized by thefact that its density is greater than that of the layer 28. Typically,the layer 30 may be made of the same material as that of layer 28;however, other suitable insulating materials may be used.

In the operation of the camera tube shown in FIG. 1, a potential ofapproximately 15 volts with respect to the cathode element 52 is appliedto the conductive layer 26 of the target member 24. The low energyelectrons emitted by the electron gun 50 are focused by the fieldelectrode 56 and the focusing means 60 onto the surface of layer 30 ofthe target member 24. As a result, the surface of the layer 30 isestablished at substantially ground potential by means of the scanningelectrons emitted by the electron gun 50. Further, a radiation imagefrom the scene 62 is projected onto the photocathode element 45 andphotoelectrons are emitted from each portion of the photocathode element45 corresponding to the amount of radiation directed thereon. Thephotoelectrons emitted by the photocathode element 45 are focused uponthe target member 24 by the focus means 60 and are accelerated to asufficiently high energy by the accelerating electrodes 46 and 48 topenetrate the insulating support layer 27 and the conductive layer 26.The incident primary electrons emitted from the photocathode element 45penetrate into the layer 28 thereby generating copious quantities of lowenergy, secondary electrons within the voids of the porous layer 28. Thelow energy electrons generated within the porous layer 28 cause theexposed surface of the target member 24 to change its potential locallydue primarily to secondary electron conduction across the layer 28 tothe conductive layer 26 and due to a finite, but much less significant,emission of transmission secondary electrons from the exposed surface ofthe layer 30 which are then collected by the electrode 64. Thus, asexplained above, a pattern of discrete charges of potential on the exitsurface of the layer 30 has been established, which may be sensed orread out by any of the several well known read out techniques. In FIG.1, there is illustrated a typical vidicon type read out assembly.

Referring now to FIG. 2, there is illustrated an electron image devicesuch as a direct view imaging tube incorporating the teachings of thisinvention. The electron image device 10 comprises a vacuum tightevacuated envelope 12 made of a suitable material such as glass. Theevacuated envelope 12 includes an elongated tubular portion 13 with aninput window 14 closing off one end of the tubular portion 13 and anoutput window 16 closing off the other end of the tubular portion 13.The windows 14 and 16 are also of a suitable material such as glasscapable of transmission of the input and output radiations. On the innersurface of the input window 14 there is provided an electricallyconductive coating 18. The coating 18 is transmissive to the inputradiation and may be of a suitable material such as tin oxide. A layer20 of a suitable photoemissive material such as cesium antimony isprovided on the coating 18. The inner surface of the output window 16 isprovided with a suitable light transmissive electrically conductivecoating 22 of a suitable material such as tin oxide and a coating 23 ofa suitable fluorescent material such as zinc cadmium sulfide depositedon the coating 22. The photoemissive layer 20 is responsive to aradiation image directed thereon from a scene and emits an electronimage corresponding to the radiation image. The fluorescent material oflayer 23 emits light in the visible region in response to the electronbombardment.

Positioned between the input window 14 and the output window 16 are aplurality of target members 24. The number of target members 24 desireddepends on the intensification and/or enhanced contrast desired and onlyone may be required in some applications as for simple storageapplications. The structure of the target member 24 has been describedabove with respect to FIGS. 1 and 3.

A suitable accelerating voltage is provided between the photoemissivelayer and the first target member 24 by means of a potential source 31connected between the conductive coatings l8 and conductive layer 26. Anaccelerating voltage is also provided between the adjacent targetmembers 24 by a suitable potential source 33 connected between theconductive layer 26. A potential source 35 connected between theconductive layer 26 of the last target member 24 and the conductivecoating 22 provides the necessary acceleration voltage between the lasttarget member 24 and the output fluorescent layer 23. Further, asuitable focusing system may be provided around the envelope 12 forfocusing the electron beams between the electrodes within the envelope12. In a specific embodiment, the focusing means is shown as a permanentmagnet 34 to provide a longitudinal field within the envelope 12.

In the operation of the device as shown in FIG. 2, an image from thescene 15 is.projected onto the photoemissive layer 20 which in turngenerates an electron image corresponding to the scene image and theelectrons are accelerated by a positive potential of about 4 kilovoltsprovided by the source 31 to bombard the first target member 24. Theelectrons emitted from the photoemissive layer 20 are accelerated withsuffcient energy to penetrate the insulating support layer 27 and theconductive layer 26 into the region of layer 28 where the incidentprimary electrons generate copious numbers of secondary electrons. As isset out in the aforementioned U.S. Pat. No. 3,128,406, the porous natureof the layer 28 is a significant factor in obtaining a high gain targetmember. The electrons emitted from the first target member 24 are thenaccelerated to the second target member 24 where the electrons formingthe electron image are again multiplied. The electrons emitted from thelast target member 24 are accelerated by the potential source 35 ofabout 5 kilovolts to the surface of the layer 23, which in turn emitslight corresponding to the radiation image directed onto the inputwindow 14.

A specific example of a suitable target member 24 as shown in FIGS. 1,2, and 3 and a method of forming such a structure will now be described.In an illustrative method of manufacture, the insulating support layer27 may be formed by first oxidizing a plate of aluminum and then etchingthe aluminum plate away to leave the support layer 27 ofa suitablethickness of about 1,000 angstroms. Next, the conductive layer 26 isformed by evaporating aluminum onto the support layer 27 to a depth ofapproximately 1,000 angstroms. The conductive coating 26 and the supportlayer 27 are then placed in a bell jar having an atmosphere ofapproximately I millimeter of a suitable inert gas such as argon ornitrogen. A predetermined amount (such as 25 milligrams) of a suitablematerial such as potassium chloride is evaporated at a distance ofapproximately 3 inches onto the conductive layer 27. The evaporationprocess is carried out at a temperature slightly in excess of themelting point of the potassium chloride. The potassium chloride isevaporated to completion and it is found that the density of the layer28 is approximately 1 to 10 percent of its bulk density and has athickness of approximately 20 microns. In order to produce the desiredsecondary emission of electrons within the voids of the materialcomprising layer 28, layer 28 should have a mass per unit area in theapproximate range of from 25 micrograms per square centimeter to 200micrograms per square centimeter and a thickness of from 10 to 30microns. Next, a second layer 30 of a suitable material such aspotassium chloride may be deposited by evaporation in an inertatmosphere and at a pressure of approximately 01 millimeters of mercuryor less. It is a significant aspect of this invention that a secondregion or layer 30 of the target member 24 have a denser structure thanthe region or layer 28 adjacent to the conductive layer 26. In theillustrative method described above, this may be achieved by evaporatingthe potassium chloride in an atmosphere of substantially reducedpressure as compared to that pressure at which layer 28 was deposited.In order to effectively control the equilibrium potential which may beestablished on the surface of layer 30 without seriously reducing theperformance characteristics of the target, layer 30 should have a massper unit area in the approximate range of 5 to 25 micrograms per squarecentimeter and a thickness in the approximate range of 0.5 to 5.0microns. The density of the layer 30 lies in the approximate range of 10to 50 percent of its bulk density.

As explained above, the primary electrons that penetrate into the porousor spongy layer 28 create a number of free electrons with low energy.Part of the low energy secondary electrons generated by primaryelectrons in the porous layer 28 are able to escape from the targetmember 24 thereby leaving a positive charge on the surface of the layer30. As this process continues, the surface charge (Q) will continue torise due to the very high resistivity of the layers 28 and 30. Inaddition, a potential difference (V) is developed across the layers 28and 30 according to the relationship: V= Q/C, where C is the aggregatecapacity of the layers 28 and 30 relative to the conductive layer 26. Ithas been observed that increases in the potential difference (V)increases the amount of secondary electron current by enhancing theescape probability for free electrons formed within the layer 28. When apotential difference (V) above a predetermined value is establishedacross the layers 28 and 30 by the effect described above, conductionthrough the layers 28 and 30 takes place since free electrons and/orconduction electrons are made available by the impact of the high energy(5 to l0 KV) write electron beam. This mun-s conduction limits theamotirTci positive charge that can be established upon the surface ofthe target member 24. Equilibrium between the charging of the surface bysecondary emission and discharge by conduction is obtained whenever thesecondary emission conduction current equals the transmission secondaryemission current. This condition corresponds to a voltage across thelayers 28 and 30 which may be defined as the equilibrium voltage(V,,,,,,).

The equilibrium potential (V of typical target members is found to behigher than the voltage required to cause breakdown of the layers 28 and30. As a result, the exit surface of the target member 24 may continueto go positive by the process described above and attain a levelexceeding the breakdown of the target member 24. However, theequilibrium potential (VF may be lowered by decreasing the transmissionsecondary emission current and/or increasing the secondary emissionconduction current. According to this invention, this may be easilyaccomplished by providing a second layer or region of a denserconsistency over the very porous layer or region in which the secondaryelectrons are generated. In terms of the physical phenomena, it may beunderstood that a denser layer or region may effectively decrease theprobability of secondary electrons escaping from the exit surface of thetarget member and, at the same time, increase the solid state conductionthrough the target member. This reduces the dependence of theequilibrium potential on the external electric field so that anauxiliary grid to limit the external electric field may be eliminated.

It is noted that it is not necessary to form a target member having twodiscrete layers as shown in FIG. 3. Instead, a target member 74 such asis shown in FIG. 4 may be provided having a single, secondary emissivelayer 72 wherein that portion or region adjacent to a surface 78abutting an electrically conductive layer 76 has a density ofapproximately 1 to percent of the bulk density and that region close toan exit surface 80 of the layer 72 has a density of about 30 percent ofthe bulk density. This type of target structure will not only reduce theamount of transmission secondary electrons escaping from the exitsurface of the target member but it will also increase the secondaryemission conduction current through the layer 72 which further tends toreduce the equilibrium potential V A specific example of the targetmember 74 is shown in FIG. 4 and a method of forming such a structurewill now be described. An insulating support layer 77 is formed ofaluminum oxide and the conductive layer 76 is evaporated thereon asdescribed above. Then the secondary emissive layer 72 of a suitablematerial such as potassium chloride is evaporated upon the conductivelayer 76. Specifically, the material is evaporated at a distance ofapproximately 3 inches onto the conductive layer 76. The evaporationprocess is conducted at a temperature slightly in excess of the meltingtemperature of potassium chloride in a suitable inert gas atmospheresuch as argon. Initially, the evaporation is conducted at a pressure ofapproximately 2 to 3 millimeters of mercury. As the evaporationproceeds, the pressure of the inert atmosphere is continually reduced bypumping out the inert atmosphere so that at the end of the evacuationprocess, the remaining pressure is in the order of a tenth ofamillimeter of mercury or less. Thus, it may be understood that thoseportions of the layer 72 initially deposited upon the conductive layer78 are very porous and that as the process of evaporation continues thatthe regions of the layer 72 become increasingly more dense.

In addition, it is not necessary to form the second layer of the targetmember of the same material as the first layer deposited directly uponthe conductive layer. In a particular application related to cameratubes as shown in FIG. 1, it may be desired to select a differentmaterial having less gain and a first crossover potential (V inreflection greater than that of the material deposited directly upon theconductive layer. Referring now to FIG. 5, a target member 84 is formedwith an insulating support layer 87 and a conductive layer 90 depositedthereon. Further, a secondary emissive material similar to thosematerials described may be evaporated to form a porous layer 88 upon theconductive layer 90. Next, a layer 86 of a suitable electricallyconductive material such as aluminum and having substantially percent ofits bulk density may be evaporated thereon in a near vacuum condition toform a thin discontinuous coating to a depth of only a few molecules(Le, 10 to 20 angstroms) upon the particles of layer 88. It is notedthat the conductive material not only coats the surface of layer 88 buttends to penetrate within the porous layer 88 and coat the voids tothereby enhance the conduction across ,this layer. Such a target member84 would not only have the characteristic that the equilibrium potentialis below the breakdown potential, but may also exhibit thecharacteristic that the equilibrium potential is less than the firstcrossover potential in reflection.

Thus, it may be seen that there has been disclosed a target member foran electron image device which has an equilibrium potential below thatpotential which would cause the breakdown of the target member. Further,it is obvious that the auxiliary grid electrodes required by the devicesof the prior art to limit the potential of the exit surface of thetarget member may according to this invention be eliminated. Inaddition, there has been shown a target member which may be easilymanufactured and incorporated within electron imaging devices withoutthe necessity of incorporating additional potential sources associatedwith the above-described auxiliary grids.

While there has been shown and described what are presently consideredto be the preferred embodiments of this invention, modifications theretowill readily occur to those skilled in the art. It is not desired,therefore, that the invention be limited to the specific arrangementsshown and described and it is intended to cover in the appended claimsall such modifications as fall within the true spirit and scope of theinvention.

We claim as our invention:

1. A target member for an electron image device comprising anelectrically conductive member, a first region disposed on said memberand comprised of a porous insulator material having the property ofgenerating secondary electrons within the volume of said material inresponse to a bombardment of primary electrons and of conducting saidsecondary electrons through the voids of said porous insulator material,and a second region disposed on said first region having a densitygreater than that of said first region but not in excess of 50 percentof the density of said second region material in the bulk form.

2. A target member for an electron image device comprising a conductivemember, a first layer being disposed on said member in a porous form andhaving the property of generating secondary electrons within the volumeof said first layer in response to a bombardment of primary electrons,said first layer having a density in the range of l to 10 percent of thedensity of said material in the bulk form, and a second layer disposedon said first layer and having a density substantially greater than saidfirst layer and in the range of 10 to 50 percent of the density of saidsecond layer material in the bulk form to impede the escape of secondaryelectrons from said target member.

3. A target member for an electron image device comprising anelectrically conductive member, a first layer disposed on said member ina porous form with a mass per unit area in the approximate range of 25to 200 micrograms per square centimeter and a thickness in theapproximate range of 10 to 30 microns and having the property ofgenerating secondary electrons within the voids of said first layer inresponse to a bombardment of primary electrons, and a second layerdisposed on said first layer and having a mass per unit area in theapproximate range of 5 to 25 micrograms per square centimeter, and athickness in the approximate range of 0.5 to 5.0 microns, said secondlayer limiting the probability of escape of said secondary electronsfrom said first layer to thereby lower the equilibrium potentialestablished upon the surface of said second layer.

lOl032 054l 4. A target member for an electron image device comprisingan electrically conductive member, and a layer disposed on said member,said layer having a first surface adjacent said member and a secondsurface remote from said member, said layer having a density of a valueless than 10 percent of the density of the material of said layer in thebulk form at said first surface and the density of said layer becomingprogressively greater toward said second surface said layer made of aninsulating material having the property of generating electrons inresponse to electron bombardment and of supporting the conduction ofelectrons through the voids of said layer.

5. A target member for an electron image device comprising anelectrically conductive member, and a layer of insulating materialhaving the characteristic of generating secondary electrons within thevolume of said layer in response to the bombardment of primaryelectrons, said layer disposed on said member in a manner that thedensity of said layer progressively increases across the thickness ofsaid layer, a first region of said layer adjacent said member having adensi ty in the range of l to 10 percent of the density of said materialin the bulk form, a second region of said layer having a density inexcess of 10 percent of the density of said material in the bulk formsufficient to thereby impede but not totally suppress the escape ofsecondary electrons from the layer and to increase the solid stateconduction of electrons across said layer.

6. An electrical device comprising a target member including anelectrically conductive element, and a layer of secondary emissivematerial deposited on said conductive element, said layer having a firstsurface contiguous to said conductive element and a second surfaceremote from said conductive element, said layer having a density thatprogressively varies from a minimum value in the range of l to 10percent of the density of said material in the bulk form at said firstsurface to a maximum value at said second surface, said maximum valuebeing several times greater than said minimum value of said density;means for directing an electron image onto the exposed surface of saidconductive element; and a display means disposed to receive thesecondary electrons emitted by said target member and to provide anintensified visible image of said electron image.

7. A target member for an electronic image device comprising anelectrically conductive member, a porous first layer disposed on saidconductive member and made of an insulating material for generatingsecondary electrons within the volume of said material in response to abombardment of primary electrons, said first layer having a density inthe approximate range of 1 to 10 percent of the density of said materialin the bulk form, and a second discontinuous layer disposed on saidfirst layer of an electrically conductive material, said conductivematerial having a density substantially that of the density of saidconductive material in the bulk form to thereby increase the firstcrossover potential and reduce the equilibrium potential of said targetmember.

8. An electron image device comprising a target element including aconductive member, a first layer disposed on said conductive member inporous form having a density less than 10 percent of the density of thefirst layer material in the bulk form and having the property ofgenerating and conducting secondary electrons through the voids of saidfirst layer, and a second layer disposed on said first layer having adensity substantially greater than first layer in the approximate rangeof 10 to percent of the density of the second layer material in the bulkform for limiting the escape of said secondary electrons from said firstlayer; and means for directing electron through said conductive memberand into the voids of said first layer.

9. An electronic camera device comprising a target member including anelectrically conductive member, a first layer of a material having theproperty of generating secondary electrons within the volume of saidmaterial in response to a bombardment of primary electrons and beingdeposited on said conductive member in a porous form having a densityless than 10 percent of the density of said material in the bulk form,and a second la er being disposed on said first layer and having adensity subs antially greater than said first layer and in theapproximate range of 10 to 50 percent of the density of said secondlayer material in the bulk form; a first source for directing primaryelectrons on said conductive member and into the volume of said firstlayer to establish a charge pattern representative of said primaryelectrons; and a second source for directing electrons onto said exposedsurface of said second layer to derive an electrical signal therefromrepresentative of said charge pattern.

10. An image intensifying device comprising at least one target memberincluding an electrically conductive member, a first layer of aninsulating material having the property of generating secondaryelectrons within the volume of said material and being deposited on saidconductive member in a porous form having a density in the approximaterange of l to 10 percent of the density of said material in the bulkform, and a second layer disposed on said first layer and having adensity substantially greater than said first layer and in theapproximate range of between 10 to 50 percent of the density of saidsecond layer material in the bulk form; means for directing an image ofprimary electrons through said conductive member and into said firstlayer; and display means for providing a visible image in response tosecondary electrons received from said target member.

11. A target element for an electron image device comprising anelectrically conductive member, and a storage member including a firstlayer of an insulating material deposited on said conductive member in aporous form with a density in the approximate range of l to 10 percentof the density of said material in the bulk form, said insulatingmaterial having the properties of generating secondary electrons withinthe voids of said material in response to a bombardment of primaryelectrons and of conducting said secondary electrons through the voidsof said insulating material, and means for impeding the escape of saidsecondary electrons from said storage member and for enhancing the solidstate conduction of electrons through said storage member including asecond layer disposed on said first layer and having a densitysubstantially greater than said first layer and in the approximate rangeof 10 to 50 percent of the density of said second layer material in thebulk form.

12. A target member for an electron image device comprising anelectrically conductive member, and a storage member including a firstlayer of a porous material deposited on said conductive member with adensity in the: approximate range of l to 10 percent of the density ofsaid material in its bulk form and having the property of generatingsecondary electrons within the voids of said porous material in responseto a bombardment of primary electrons, and means for impeding the escapeof secondary electrons from said storage member and for increasing thefirst crossover potential of said storage member including a second,discontinuous layer of a conductive material disposed to depth of a fewmolecules on said first layer.

1. A target member for an electron image device comprising an electrically conductive member, a first region disposed on said member and comprised of a porous insulator material having the property of generating secondary electrons within the volume of said material in response to a bombardment of primary electrons and of conducting said secondary electrons through the voids of said porous insulator material, and a second region disposed on said first region having a density greater than that of said first region but not in excess of 50 percent of the density of said second region material in the bulk form.
 2. A target member for an electron image device comprising a conductive member, a first layer being disposed on said member in a porous form and having the property of generating secondary electrons within the volume of said first layer in response to a bombardment of primary electrons, said first layer having a density in the range of 1 to 10 percent of the density of said material in the bulk form, and a second layer disposed on said first layer and having a density substantially greater than said first layer and in the range of 10 to 50 percent of the density of said second layer material in the bulk form to impede the escape of secondary electrons from said target member.
 3. A target member for an electron image device comprising an electrically conductive member, a first layer disposed on said member in a porous form with a mass per unit area in the approximate range of 25 to 200 micrograms per square centimeter and a thickness in the approximate range of 10 to 30 microns and having the property of generating secondary electrons within the voids of said first layer in response to a bombardment of primary electrons, and a second layer disposed on said first layer and having a mass per unit area in the approximate range of 5 to 25 micrograms per square centimeter, and a thickness in the approximate range of 0.5 to 5.0 microns, said second layer limiting the probability of escape of said secondary electrons from said first layer to thereby lower the equilibrium potential established upon the surface of said second layer.
 4. A target member for an electron image device comprising an electrically conductive member, and a layer disposed on said member, said layer having a first surface adjacent said member and a second surface remote from said member, said layer having a density of a value less than 10 percent of the density of the material of said layer in the bulk form at said first surface and the density of said layer becoming progressively greater toward said second surface said layer made of an insulating material having the property of generating electrons in response to electron bombardment and of supporting the conduction of electrons through the voids of said layer.
 5. A target member for an electron image device comprising an electrically conductive member, and a layer of insulating material having the characteristic of generating secondary electrons within the volume of said layer in response to the bombardment of primary electrons, said layer disposed on said member in a manner that the density of said layer progressively increases across the thickness of said layer, a first region of said layer adjacent said member having a density in the range of 1 to 10 percent of the density of said material in the bulk form, a second region of said layer having a density in excess of 10 percent of the density of said material in the bulk form sufficient to thereby impede but not totally suppress the escape of secondary electrons from the layer and to increase the solid state conduction of electrons across said layer.
 6. An electrical device comprising a target member including an electrically conductive element, and a layer of secondary emissive material deposited on said conductive element, said layer having a first surface contiguous to said conductive element and a second surface remote from said conductive element, said layer having a density that progressively varies from a minimum value in the range of 1 to 10 percent of the density of said material in the bulk form at said first surface to a maximum value at said second surface, said maximum value being several times greater than said minimum value of said density; means for directing an electron image onto the exposed surface of said conductive element; and a display means disposed to receive the secondary electrons emitted by said target member and to provide an intensified visible image of said electron image.
 7. A target member for an electronic image device comprising an electrically conductive member, a porous first layer disposed on said conductive member and made of an insulating material for generating secondary electrons within the volume of said material in response to a bombardment of primary electrons, said first layer having a density in the approximate range of 1 to 10 percent of the density of said material in the bulk form, and a second discontinuous layer disposed on said first layer of an electrically conductive material, said conductive material having a density substantially that of the density of said conductive material in the bulk form to thereby increase the first crossover potential and reduce the equilibrium potential of said target member.
 8. An electron image device comprising a target element including a conductive member, a first layer disposed on said conductive member in porous form having a density less than 10 percent of the density of the first layer material in the bulk form and having the property of generating and conducting secondary electrons through the voids of said first layer, and a second layer disposed on said first layer having a density substantially greater than first layer in the approximate range of 10 to 25 percent of the density of the second layer material in the bulk form for limiting the escape of said secondary electrons from said first layer; and Means for directing electron through said conductive member and into the voids of said first layer.
 9. An electronic camera device comprising a target member including an electrically conductive member, a first layer of a material having the property of generating secondary electrons within the volume of said material in response to a bombardment of primary electrons and being deposited on said conductive member in a porous form having a density less than 10 percent of the density of said material in the bulk form, and a second layer being disposed on said first layer and having a density substantially greater than said first layer and in the approximate range of 10 to 50 percent of the density of said second layer material in the bulk form; a first source for directing primary electrons on said conductive member and into the volume of said first layer to establish a charge pattern representative of said primary electrons; and a second source for directing electrons onto said exposed surface of said second layer to derive an electrical signal therefrom representative of said charge pattern.
 10. An image intensifying device comprising at least one target member including an electrically conductive member, a first layer of an insulating material having the property of generating secondary electrons within the volume of said material and being deposited on said conductive member in a porous form having a density in the approximate range of 1 to 10 percent of the density of said material in the bulk form, and a second layer disposed on said first layer and having a density substantially greater than said first layer and in the approximate range of between 10 to 50 percent of the density of said second layer material in the bulk form; means for directing an image of primary electrons through said conductive member and into said first layer; and display means for providing a visible image in response to secondary electrons received from said target member.
 11. A target element for an electron image device comprising an electrically conductive member, and a storage member including a first layer of an insulating material deposited on said conductive member in a porous form with a density in the approximate range of 1 to 10 percent of the density of said material in the bulk form, said insulating material having the properties of generating secondary electrons within the voids of said material in response to a bombardment of primary electrons and of conducting said secondary electrons through the voids of said insulating material, and means for impeding the escape of said secondary electrons from said storage member and for enhancing the solid state conduction of electrons through said storage member including a second layer disposed on said first layer and having a density substantially greater than said first layer and in the approximate range of 10 to 50 percent of the density of said second layer material in the bulk form.
 12. A target member for an electron image device comprising an electrically conductive member, and a storage member including a first layer of a porous material deposited on said conductive member with a density in the approximate range of 1 to 10 percent of the density of said material in its bulk form and having the property of generating secondary electrons within the voids of said porous material in response to a bombardment of primary electrons, and means for impeding the escape of secondary electrons from said storage member and for increasing the first crossover potential of said storage member including a second, discontinuous layer of a conductive material disposed to depth of a few molecules on said first layer. 